The impact of such a move on Shell's 380,000 barrel-a-day oil output from the Warri region was unclear, because evacuation did not always mean halting production, an industry source told Reuters.

About three-quarters of the Warri region's output was from the Niger delta swamps where militants can easily approach production platforms by boat, the report said.

Shell, the largest oil producer in Nigeria, has suffered from a series of recent attacks in that country.

Shell pulled out about 330 workers from four platforms in the swamps after an assault on the Benisede flow station in which at least six people were killed.

Last Friday, Shell declared force majeure on 106,000 barrels per day of crude oil exports from Nigeria on after an attack damaged a major pipeline. This released the company (from its contractual export commitments while repairs to the Trans-Ramos pipeline, expected to take 3-4 days, were undertaken.

Shell's Nigerian production has dropped by more than 10% since the attacks began in December.

Kidnappings and pipeline explosions are common in the Niger Delta.

Local groups complain they do not see the benefits of the area's oil wealth and are accusing Shell of reneging on promises to undertake development projects.

Poverty remains wdiespread in Nigeria, despite the country being Africa's largest and the world's eighth-largest oil producer.